Method for melting and cracking amorphous polyolefin

ABSTRACT

Melting and cracking of amorphous polyolefin are carried out with a relatively small quantity of heat consumption and also with easiness in separating sludge, in converting it into oil by maintaining amorphous polyolefin at a high temperature of about 130° - 300°C for a relatively long time alone or together with a petroleum hydrocarbon oil to reduce the viscosity of the resultant melt to 5 - 250 CP, spraying the resultant melt upon lumps of amorphous polyolefin, separating impurities in the lumps as sludge, cracking the resultant molten mixture at a temperature of about 250° - 450°C and recovering oily products. For that purpose an apparatus comprising a combination of a holding tank for melt of amorphous polyolefin, a vessel for melting amorphous polyolefin equipped with a perforated plate or a wire-netting, a revolving shaft, a stirrer, etc. and a thermally cracking vessel equipped with a stirrer, a distillation means, and auxiliary means such as heating furnaces, pumping means for the melt and heating medium, is used.

DESCRIPTION OF THE INVENTION

This invention relates to a process for converting a solid or a semi-solid amorphous polyolefin resin into a liquid oily matter rich in olefin by melting and thermally cracking it and an apparatus useful in said process.

With the recent increase of the production of various polyolefin plastics, the treatment of scraps and by-products thereof has become a big problem requiring solution. For example, in the production of crystalline polypropylene, there are produced not only scraps of crystalline polyolefin itself but also a considerable amount of amorphous polyolefin is produced as a by-product, giving manufacturer troublesome problem of disposal. On this account various attempts have been proposed such as use of a specific catalyst capable of reducing its production as much as possible; use of by-product, amorphous polyolefin as an oil-absorbing agent; use as a land reclaiming material, etc. However, the amount consumed in these usages is exceedingly small compared with the amount of amorphous polypropylene inevitably produced with the ever growing tremendous amount of crystalline polypropylene and hence amorphous polypropylene is still a troublesome by-product which gives a difficulty for disposal. In cases of other commercial polyolefins, e.g. various polyethylene, there is similar problem for disposal of by-products.

One approach to the solution of this problem is an attempt to thermally crack solid resinous by-products into lower molecular weight oily matter and it is partly used already in practice. However an efficient apparatus which enables to conduct a continuous operation for a long period of time has not been developed until now. There has been heretofore proposed an apparatus consisting of a melting vessel which melts polyolefin by heating into liquid state and a cracking vessel which cracks resultant liquid polyolefin in order to be used for such purpose. However, the amorphous polyolefin by-produced in polyolefin plants contains usually mixed impurities which tends to form sludge in various apparatus, which is very difficult to separate. There are also other disadvantages for this attempt, such as need of large-sized transportation pumps for molten liquid, a large quantity of heat per ton for converting into oil.

An object of the present invention is to provide a method for melting and cracking amorphous polyolefin in which separation of sludge from melt is easy. Another object of the present invention is to provide a method for melting and thermally cracking amorphous polyolefin with a relatively small quantity of heat in converting it into oil. The above-mentioned object can be attained by the method and apparatus of the present invention.

The method of the present invention i.e. a melting and thermally cracking method of amorphous polyolefin which is characterized in maintaining amorphous polyolefin alone or together with a petroleum hydrocarbon oil, at a temperature of about 130°C to 300°C for a sufficiently long time to reduce the viscosity of the resulting melt down to 5 - 250 CP, spraying the melt upon lumps of amorphous polyolefin piled on a flat perforated structure in such a way that the proportion of the amount by weight of the melt to that of the lumps is at least 5 to promote the heat transmission from the melt to the lumps and to melt the lumps of amorphous polyolefin into liquid state, separating unmelted impurities in the lumps in the form of sludge, cracking the resulting separated liquid at a temperature of about 250° - 450°C and recovering an oily product.

The apparatus of the present invention is a melting and thermally cracking apparatus of amorphous polyolefins which is characterized by being provided with as principal members, a holding tank of melt of amorphous polyolefin in order to reduce the viscosity of the melt 5 - 250 CP by maintaining said amorphous polyolefin alone or together with a petroleum hydrocarbon oil, at a temperature of about 130° - 300°C, a vessel for melting lumps of amorphous polyolefin, equipped with a flat perforated structure such as a perforated plate or wire-netting for loading the lumps, fixed at a position between the top and the bottom of the vessel, a revolving shaft which penetrates the flat perforated structure at its center, a stirrer close to the bottom of the vessel, a charging port of the lumps, a shower means for spraying the melt of amorphous polyolefin sent from the holding tank, upon the lumps, a funnel-like structure for promoting convection of heat and evaporation of moisture, a discharging port of the resulting melt and a take-out port of sludge at the bottom of the vessel, a cracking vessel equipped with a stirrer, which receives melt of amorphous polyolefin and cracks it at a temperature of 250° - 450°C, a distillation means for distilling the resultant cracked liquid and gas, a heating furnace for melt of amorphous polyolefin, a heating furnace for heating the melt in the cracking furnace, a heating furnace of heating medium used for heating various means, and pumping and piping means for melt of amorphous polyolefin.

The process and the apparatus of the present invention will be more fully understood by the following detailed description which is made referring to the attached drawings in which:

FIG. 1 is a flow-sheet for carrying out the process of the present invention and

FIG. 2 is a schematic view of a vertical section of the melting vessel used in the method of the present invention.

FIG. 3 is a schematic view of a vertical section of the cracking vessel used in the method of the present invention.

Referring at first to FIG. 1 which is a flow sheet of the apparatus of the present invention, heavy oil is filled in holding tank E, sent to heating furnace B by a pump. The temperature of oil is elevated to a temperature of 130° to 300°C depending upon the kind and molecular weight of polyolefin.

From hopper 3 or 3' for charging, lumps of amorphous polyolefin are charged into melting vessel A₁ or A₂ (the detail of which is shown in FIG. 2 and hence explanation will be made later referring to FIG. 2) and deposited on perforated plate 1 situated in the middle part of the vessel. The cover of hopper 3 or 3' is closed automatically, the heavy oil from holding tank E is passed through heating furnace B to raise its temperature to about 130° - 350°C, then through pipe 4 provided in the inside of and near the top of vessel A₁ or A₂ and sprayed from nozzel 5 over the above-mentioned deposited lumps or amorphous polyolefin to melt the latter. Heavy oil which has melted the amorphous polyolefin is passed through filter tank X and returned to holding tank E to keep its constant temperature of 130° - 300°C and its constant viscosity of about 5 - 250 CP. The heavy oil containing amorphous polyolefin is sent to cracking vessel F where it is recycled to heating furnace C, heated to about 250°- 450°C and amorphous polyolefin included therein is cracked. The liquid containing thermally cracked gas is passed through the heat exchanger coil of holding tank E, supplying heat to molten liquid in said tank E and is passed through buffer tank D, then sent to distillation apparatus G, condensed in condenser H and separated into products of several fractions and a heavy oil. Thus the heavy oil is used only during the starting time in holding tank E. The content of holding tank E becomes gradually molten amorphous polyolefin alone having a viscosity of 5 - 250 CP with the increase of the amount of new amorphous polyolefin charged.

In addition, in cases where an urgent treatment is required or where raw material having extremely high molecular weight is used, above procedure is often repeated with addition of petroleum hydrocarbon oil at an appropriate time to keep the viscosity of the molten liquid at 5 - 250 CP. It is necessary to select a residence time and hold the molten liquid of amorphous polyolefin which is to be sprayed in the above-mentioned melting vessel A₁ and A₂ for a sufficiently long time (usually about 15 hours) to make the viscosity of molten liquid fall in the range of 5 - 250 CP. However, as a special occasion if petroleum hydrocarbon oil is added, the above-mentioned holding time can be shortened. Even when molten liquid is brought to the state containing no more heavy oil after continuation of operation, the molten liquid is sprinkled in melting vessels A₁ and A₂ in order to dissolve amorphous polyolefin. The amount of the molten amorphous polyolefin whose viscosity is thermally degraded to 5 - 250 CP or the amount of the molten amorphous polyolefin which is admixed with petroleum hydrocarbon oil is about 5 - 13 times the weight of solid or semifluid solid polyolefin lumps which is to be melted. The subsequent procedures are carried out in the same manner as in case where heavy oil is incorporated.

The ratio of conversion to liquid oil in the process of the above-mentioned embodiment of the present invention is 70 - 80% of the amorphous polyolefin used. Since the heat quantity required per T of product is 300,000 - 400,000 K caL and since the heat quantity required in case of conventional manner of heating carried out with a direct-firing coiled pipe is 500,000 K cal, notable saving in fuel consumption can be attained. In addition, compared with conventional process, the process of the present invention is less dangerous and easier for separating sludge. Moreover, since molten liguid of amorphous polyolefin is handled with a reduced viscosity, installations such as pumps, etc. become smaller size, leading to saving of electricity.

The product oil can be used as fuel oil, solvent and lubricating oil, and also used as raw material for paints after partial polymerization. The amount of sludge formed in cracking vessel F is about 5% by weight of the total amount. The sludge is a material rich in free carbonaceous matter. It is transferred to sludge tank Q by pumps and adequately admixed with light cracked oil or heavy cracked oil to be sold as fuel. It has a calorific value of about 11,000 Kcal/Kg. Sludge is produced also in holding tank E, but it is usually sent to the melting vessel. The amount of the sludge withdrawn from the melting vessel is about 5 - 10% of the total amount but the sludge is passed through strainer K and sent to sludge tank L, then supernatant liquid thereof is returned to the melting vessel. From the top of melting vessel of amorphous polyolefin A₁ and A₂, holding tank E and distillation apparatus G, exhaust gas containing water vapor and low boiling point materials is vented. The exhaust gas is pased through oil-water separator I, and water is recovered by passing through active carbon vessel V to be used again. If desired it is used as cooling water for condenser H.

Uncondensable gas is passed through gas tank T (2 - 6% of the total) and burnt is flared slag U.

A following example is given to illustrate the method and the apparatus of the present invention which is, however, not intended to limit the present invention.

EXAMPLE

In this example, amorphous polypropylene is used as an amorphous polyolefin, but similarly the process and the apparatus of this invention are also applicable to other amorphous polyolefins after slightly modifying the temperature. Lumps of amorphous polypropylene having a viscosity of 1200 CP in the state of molten liquid at 130°C are charged into melting vessel A₁ or A₂ from hopper 3 or 3' at a rate of 1 T/hr and deposited on perforated plate 1. 35 Kl of petroleum heavy oil having boiling point higher than 270°C is charged into holding tank E which has a radius of 1422 mm a length of 7220 mm and a volume of 38 Kl, then the heavy oil is circulated between holding tank E and heating furnace B by a pump at the rate of 25 m³ /hr and its temperature is raised to 260°C. The heavy oil is sent to shower pipe 4 of either of two melting vessel A₁ or A₂ having the same shape and the same volume and sprinkled over the amorphous polypropylene deposited on preforated plate 1 from nozzle 5. Charged amount of amorphous polypropylene is 5 m³ at a bulk density of 0.2 per one dissolving vessel and treated amount is 5 m³ /hr. The amount of amorphous polypropylene on perforated plate 1 can be watched externally by level meter 10. Molten liquid containing dissolved amorphous polypropylene flows down from perforated plate 1 and further flows on guide funnel 8 of molten liquid as thin film while evaporating water and lower boiling materials and molten liquid containing sludge is guided downward to the lower part of melting vessel A₁ or A₂ and is sent to the bottom of melting vessel A₁ or A₂ in order to smooth the flow of heat by convection. With regard to the prevention of sludge from adhering, stirring is continued at the lower part with stirring blade 6 which is slowly driven by shaft 2.

First, sludge is carried to the bottom of melting vessel A₁.sup.. A₂ automatically by molten liquid guide funnel 8 and is withdrawn from sludge discharge port 7, then is passed through strainer K and separated at sludge tank L. The amount of sludge which varies depending upon raw material polypropylene is about 5 - 10%. The lower part of melting vessel is covered with mantles over the area of 8,487 m² in order to keep a temperature by using a heating medium oil having been heated at heating furnace Y and sent from heating medium oil tank Z which is maintained at a temperature.

Supernatant melt of amorphous polypropylene is sent from discharge port 9 for supernatant molten liquid via filter tank X to holding E. For a while from starting there is amorphous polypropylene dissolved in heavy oil in holding tank E, but since heavy oil is used only for a while at starting time and since it is removed at the distillation apparatus, operation is continued in the state amorphous polypropylene alone, in the normal operation for a long time. The molten polypropylene is delivered to a position close to the bottom of cracking vessel F (refer to FIG. 3, volume is about 18 m³) equipped with stirrer 16. On the other hand, a portion of molten liquid in the tank is withdrawn from a position near the bottom and is sent to heating furnace C to raise its temperature to 430°C - 450°C (0.5 CP) then returned to the upper part of the tank. The temperature of the liquid in cracking vessel F is kept at 400°C. Cracking vessel F is provided with an entrance for molten liquid 14 to receive molten liquid having a temperature of 260°C and also stirrer 16. Molten liquid is sent from molten liquid discharge port 15 to heating furnace C by a pump and after being decomposed partly, at a temperature of 450°C, it returns through liquid return port 11 to cracking vessel F. Further, cracking vessel F is equipped with liquid level gauge 12. By using it, operation is carried out in such a way that port 11 for liquid returning from heating furnace C is kept below the liquid level. Cracked gas in cracking vessel F is led to a coil in holding tank E to effect heat exchange and after heating indirectly the liquid in holding tank E, it is passed through buffer tank D and sent to distillation apparatus G to separate the material into fractions of lower than 300°C, from 300°C to 370+C and higher than 370°C. These fractions are received in product tanks 0₁, 0₂, 0₃, amounts of which are as follows.

    ______________________________________                                                            proportion                                                                               amount                                                               of        obtained                                          range of           decomposed                                                                               from 3500 Kg                                      fraction           material  of raw                                                                         material                                          ______________________________________                                         lower than 300°C                                                                      top gas  20%        500 Kg                                       300° - 370°C                                                                   gas oil  35%        875 Kg                                                     gas oil                                                          higher than 370°C                                                                     and      45%       1125 Kg                                                     bottom                                                           ______________________________________                                    

Noncondesable gas and lower boiling point materials generated from each apparatus are sent to oilwater separator I or gas tank T. Among these, the water from oil-water separator is passed through active carbon vessel V and collected for reuse. The amount of uncondensable gas from oil-water separator I and each apparatus is about 3%. It is burnt at flare stack U. For the temperature maintenance of each piping for heating furnace B and C to holding tank E and decomposing vessel F, heating medium oil (Nisseki high thermo 120 (trade name)) from heating medium oil tank Z is used.

Heating furnace B is used to heat the molten liquid in holding tank E and heating furnace C is used to heat the liquid in decomposing vessel F. Sludge is withdrawn from sludge discharge port 13 of cracking vessel by a pump at every ten hours and transferred to sludge tank Q. The resultant sludge is about 5% of the total. It is a material rich in free carbonaceous materials and can be sold as fuel oil. For the purpose of fuel oil, it is preferable to be mixed with a suitable proportion of light or heavy cracked oil. Sludge in melting vessel A₁.sup.. A₂ is removed through strainer K and supernatant liquid is returned to melting vessels A₁, A₂. Sludge formed in holding tank E is sent to melting vessel A₁ and A₂. Also from melting vessels A₁ and A₂, exhaust gas is discharged. It consists of steam and lower boiling point materials. The amount of the steam resulting from 3500 Kg of amorphous polypropylene is 300 Kg. 

What is claimed is:
 1. A method for melting and thermally cracking a solid, amorphous polyolefin material comprising, maintaining a melted body of amorphous polyolefin material at a temperature of about 130°C to 300°C for a period of time sufficient to reduce the viscosity thereof to about 5 to 250 cps, maintaining a bed of lumps of solid, amorphous polyolefin material on an elevated, perforated support, passing at least a portion of said melted polyolefin material of reduced viscosity through said bed of lumps of polyolefin material at a rate sufficient to maintain a weight ratio of said melted polyolefin of reduced viscosity to said lumps of polyolefin material of at least 5/1 and melt said lumps of polyolefin material, separating a liquid phase of said melted polyolefin material from a sludge phase, containing unmelted impurities, passing a portion of said liquid phase to said melted body of polyolefin to replenish the same, cracking the remainder of said liquid phase at a temperature of about 250°C to 450°C and recovering a liquid oil product from the effluent of the cracking step.
 2. A method in accordance with claim 1 wherein the method is performed continuously.
 3. A method in accordance with claim 1 wherein a heavy oil fraction is maintained at a temperature of about 130°C to 300°C and passed through the bed of lumps of polyolefin material until sufficient melted polyolefin material is produced in the melting step to maintain the melted body of polyolefin material.
 4. A method in accordance with claim 1 wherein a heavy oil fraction isat least periodically added to the melted body of polyolefin to aid in reducing the viscosity thereof.
 5. A method in accordance with claim 1 wherein at least a part of the heat necessary to maintain the temperature of the melted body of polyolefin is supplied by passing the effluent from the cracking step in indirect heat exchange through said melted body of polyolefin.
 6. A method in accordance with claim 1 wherein the total liquid phase is passed to the melted body of polyolefin and the remaining portion, not utilized to replenish said melted body of polyolefin, is then passed to the cracking step and the effluent from said cracking step is passed in indirect heat exchange and countercurrent to the flow of said liquid phase through said melted body of polyolefin.
 7. A method in accordance with claim 1 wherein the melted polyolefin of reduced viscosity is sprayed on top of the bed of lumps of polyolefin. 